There has been a strong demand for increasing the strength of auto parts, in particular constant velocity joint intermediate shafts, in recent years along with the increased output of automobile engines and environmental regulations. The strength characteristics required for constant velocity joint intermediate shafts are mainly static torsional strength and torsional fatigue strength.
As the technology relating to the drive shafts among conventional auto parts, for example, a method of production of a drive shaft obtained by forming steel has been provided that includes C: 0.30 to 0.38%, Mn: 0.6 to 1.5%, B: 0.0005 to 0.0030%, Ti: 0.01 to 0.04, and Al: 0.01 to 0.04% into a drive shaft, making the ratio of the induction hardened depth by the induction hardening and the radius of the steel material 0.4 or more, and omitting the tempering after induction hardening has been proposed (for example, see Japanese Patent Publication No. 63-62571). This method does not consider the torsional fatigue strength taken particular note of as a strength characteristic in the present invention. Further, it is estimated from the amount of carbon that there is a self-limit to the level of the increase in strength.
Further, a solid shaft for a drive shaft has been provided which includes C: 0.38 to 0.45%, Si: 0.35% or less, Mn: 0.8 to 1.5%, B: 0.0005 to 0.0035%, Ti: 0.01 to 0.05, Al: 0.01 to 0.06%, and N: 0.01% or less, having a quenched hardened layer with a surface hardness of HRC55 or more obtained by induction hardening formed so as to have a ratio of the induction hardened depth/shaft radius of 0.45 or more, and having a torsional strength of 1.47 GPa or more in terms of the maximum shear stress value has been proposed (for example, see Japanese Patent Publication No. 5-320825). However, even if using a steel material with such a composition, the strength characteristics required for a high strength part such as a constant velocity joint intermediate shaft cannot be obtained at the present. This conventional shaft includes a shaft part that has a serration shaft, but does not consider the location of fracture or the mode of fracture. Further, it is estimated from the amount of carbon that there is a self-limit to the level of the increase in strength.
Further, a high strength, high frequency quenched shaft has been provided which includes C: 0.35 to 0.70%, Si: 0.01 to 0.15%, Mn: 0.2 to 2.0%, S: 0.005 to 0.15%, Al: 0.0005 to 0.05%, Ti: 0.005 to 0.05%, B: 0.0005 to 0.005%, and N: 0.002 to 0.02% and having an average hardness in the cross-section weighted by the square of the radius (value obtained by splitting part circularly concentrically in the radial direction, weighting the hardness of the links by the square of the radius, finding the sum, and obtaining the average) of 560 or more has been proposed (for example, see Japanese Patent Publication No. 7-90484). This conventional shaft included no consideration of the location of fracture and mode of fracture. Therefore, such conventional shaft should not be said to be a high strength shaft part optimally adjusted in distribution of hardness for each location of the part.